Methods and devices for acquiring seismic data relative to an underground area beneath the sea

ABSTRACT

In order to acquire seismic data relative to an underground area beneath the sea, seismic waves are emitted in an emission direction forming an angle θ with the vertical using, at least one seismic source submerged at a depth d. A seismic signal is collected following the emission of the seismic waves and the propagation of same underground with a view to processing same. In one embodiment of the method, in order to overcome the major problem linked to the depth limit encountered by seismic sources, the processing of the seismic signal comprises a summation of a plurality of terms including the seismic signal and the seismic signal delayed by ΔT=2d·cos θ/V, in which V is the speed of propagation of the seismic waves in water.

RELATED APPLICATIONS

The present application is a National Phase entry of PCT Application No.PCT/FR2014/051139, filed May 15, 2014, which claims priority from FRPatent Application No. 13 54926, filed May 30, 2013, said applicationsbeing hereby incorporated by reference herein in their entirety.

FIELD OF THE INVENTION

The present invention relates to seismic techniques used to seek toobtain information regarding the structure and the physical propertiesof the subsoil. It relates more particularly to the implementation ofthese seismic techniques in a maritime environment.

BACKGROUND OF THE INVENTION

In a maritime environment, submerged seismic sources, such as forexample compressed-air guns, are customarily used.

The seismic wave emitted by a source submerged at a depth d propagatesin the water in a substantially spherical manner. The energy sentupwards from the source is reflected specularly at the surface and issuperimposed on the seismic energy sent downwards from the source. Thecoefficient of reflection of the seismic waves at the water-airinterface is almost equal to −1, and the reflection gives rise to achange of sign of the pressure wave. That component of the emitted fieldwhich is reflected at the surface is similar, apart from the sign, towhat would be emitted by a ghost source situated vertically in line withthe source and at a distance d above sea level.

FIG. 1 illustrates this phenomenon, with the vertical direction zdenoting the depth and the horizontal direction x a spatial coordinateparallel to the surface of the sea. The seismic signal emitted from thesubmerged source 10 in a direction forming an angle θ with the verticalhas, at the level of a wave surface Σ_(t) some distance from the source,a direct component S(t) propagated downwards from the source 10 and aghost component −S(t−ΔT) which has undergone the reflection on thewater-air surface M as if it had been emitted from the ghost source 10′.The ghost component exhibits with respect to the direct component adelay ΔT which depends on the angle θ, i.e. ΔT=2d·cos θ/V, where V isthe speed of propagation of the seismic waves in the water.

Some distance from the source, the seismic amplitude which propagatesalong the direction θ may be then written:

S ₁(t)=S(t)−S(t−ΔT)  (1)

The ghost's presence related to the surface reflection affects thespectrum of the propagated seismic signal Si. If we represent an idealsource emitting a Dirac pressure pulse, that is to say with a flatspectrum, the superposition of the reflected wave brings about:

-   -   zeros or notches in the spectrum at the frequencies which are a        multiple of 1/ΔT;    -   attenuation of the low frequencies, which is considered to be        prejudicial since the information extracted from the        measurements at the lowest frequencies is very rich, in        particular for advising regarding the speeds of propagation of        the waves in the soil.

FIG. 2 illustrates the same phenomenon as FIG. 1 in a case where θ=0,with the depth z represented by the vertical direction and the time trepresented by the horizontal direction. A Dirac pulse emitted by thesource 10 arrives at a depth z under the source at an instant t, whileits echo due to the ghost, of opposite sign, arrives at the same depth zwith the delay ΔT.

FIG. 3 shows spectra, obtained by calculation, of the signal emitted inthe vertical direction (θ=0) for source depths, d, of 5 m (curve 11), of10 m (curve 12) and of 20 m (curve 13). The shallow sources have theadvantage of rejecting the notches toward the high frequencies, while,however, attenuating the low frequencies fairly strongly.

It is possible to seek to improve the behavior at low frequencies byincreasing the depth of the source. However, the notches are then atlower frequencies. Furthermore, underwater seismic sources havediminished energy efficiencies and degraded frequency contents as thedepth increases, because of the effect of the hydrostatic pressure.

In order to regulate the emission spectrum, it is known to activateseveral sources situated at different depths. For example, in the caseof FIG. 3, the activation of the three sources at depths of 5, 10 and 20m gives rise to a spectrum represented by curve 14, resulting from thesum of the spectra represented by curves 11, 12 and 13, which shows asteeper slope at the low frequencies and zeros aligned with those of theshallowest source. This is not perfect since the resulting spectrum isnot flat. However, this is a sharp improvement. A judicious choice ofthe depths of the combined sources makes it possible to best circumventthe notches while preserving content at the very low frequencies.

A technique making it possible to put the effect of the ghost intofurther perspective consists in triggering each source placed at a givendepth at the moment at which the signal of the source situated justabove it reaches it. Thus, the primary wave field emitted downwards isput back into phase despite different source depths. Therefore, theprimary wave fields of each of the sources interfere constructivelywhereas this is not the case for the ghosts.

In another approach, the sources of one and the same set are groupedtogether in clusters each positioned at a different depth, the set ofthese clusters being triggered in a maximum timescale of a second, thusmaking it possible to preserve a stationary emission.

It has been proposed to improve the emission spectrum by disposing ascreen of gas bubbles between the source and the surface so as todecrease the reflection coefficient, thereby improving the behavior atlow frequencies and limiting the sagging of the spectrum in the notches.FIG. 4 thus shows the effect on the spectrum of a reflection coefficientr of 0.7. Curves 21, 22, 23 and 24 of FIG. 4 have been calculated withsources disposed like those which gave rise to curves 11, 12, 13 and 14in FIG. 3, respectively. It is seen that the attenuation of thereflection coefficient r boosts the lowest frequencies (A). However, thedrawback of this technique is that it is very complex to implement, andthe improvement in performance remains limited.

A major problem encountered by all the techniques proposed to dateremains the limit in terms of depth imposed on the sources, therebygreatly reducing the possibility of finding sufficient low frequenciesin the signal spectrum.

An object of the present invention is to reduce the incidence of thisproblem and more generally to improve the spectral content of theseismic signal emitted utilized in measurements performed on the basisof one or more submerged sources.

SUMMARY OF THE INVENTION

There is thus proposed a method for emitting seismic waves in a maritimeenvironment along a direction of emission forming an angle θ with thevertical, with the aid of at least one submerged seismic source, themethod comprising:

-   -   performing a first firing from a first emission position        submerged at a depth d₁; and    -   performing a second firing from a second emission position        submerged at a depth d₂, with a delay equal to (d₁+d₂)·cos θ/V        with respect to the first firing, where V is the speed of        propagation of the seismic waves in the water.

In a particular embodiment, the first and second emission positions aremerged, or situated at one and the same depth d (d₁=d₂=d) in one and thesame Fresnel zone in relation to the emitted seismic waves.

The first firing can be repeated k−1 times at the same depth or atdifferent depths, with judiciously chosen delays, k being an integergreater than 2. It is thus possible to perform an i^(th) firing from ani^(th) emission position submerged at a depth d_(i), with a delay equalto (d_(i-1)+d_(i))·cos θ/V with respect to the (i−1)* firing, for eachinteger i lying between 2 and k.

Again, the k emission positions can, without this being limiting, bemerged, or situated at one and the same depth d (d₁=d₂= . . . =d_(k)=d)in one and the same Fresnel zone in relation to the emitted seismicwaves.

In an embodiment, the direction of emission is vertical, that is to sayθ=0. The invention is then usable, in particular, to carry out verticalseismic profiles (“VSP”). However, inclined directions (θ≠0) are alsopossible.

The method rests upon a totally different approach to what has been doneor proposed up till now. The second firing is synchronized with theghost of the first firing so as to defer it in time. This amounts toplacing a virtual source at a greater depth, without having theconstraints of a more significant hydrostatic pressure.

Thus the method makes it possible to circumvent the major problemencountered hitherto, namely the maximum depth limit imposed on thesources. It is possible, just as for conventional configurations, to useseveral energy sources equivalent to that of the ghost. These sourcescan be placed, physically and/or virtually, at various depths and theycan be combined in an optimal manner.

It is not indispensable to employ sources with very fast firing rates.It is for example possible to apply a firing sequence at periodicrepetitions with the same source, on each occasion allowing the sourcetime to regain its optimal chamber pressure.

A way of reducing the firing rate consists in placing several smallsources at the same depth. This type of implementation makes itpossible, if the number of repeated firings is sufficient, to obtain anappreciable gain in terms of very low frequencies without the necessityto use a deflector between the sources and the surface.

According to another aspect, a method for acquiring seismic datarelating to a subsoil zone situated under the sea comprises:

-   -   emitting seismic waves along a direction of emission forming an        angle θ with the vertical, with the aid of at least one        submerged seismic source;    -   gathering a seismic signal subsequent to the emission of the        seismic waves and to their propagation in the subsoil; and    -   processing the seismic signal.

According to this method for acquiring seismic data, the emission of theseismic waves comprises k successive firings, where k is an integergreater than 1, including a first firing performed at a depth d₁ and k−1subsequent firings at respective depths d₂, . . . d_(k), and for eachinteger i lying between 2 and k, the ith firing is performed with adelay of (d_(i-1)+d_(i))·cos θ/V with respect to the (i−1)^(th) firing,where V is the speed of propagation of the seismic waves in the water.

In this mode of acquisition of the seismic data, the deletion of theghost occurs at the level of the emission of the seismic waves, with theaid of the emission method set forth above.

An advantageous embodiment consists in applying the same principle ofdeleting the ghost not at the level of the emission but at the level ofthe processing of the signal on the basis of a single firing or of arestricted number of firings.

There is thus proposed a method for acquiring seismic data relating to asubsoil zone situated under the sea, the method comprising:

-   -   emitting seismic waves along a direction of emission forming an        angle θ with the vertical, the emitted seismic waves comprising        at least one emission sequence generated with the aid of at        least one submerged seismic source, each emission sequence        having an associated depth;    -   gathering a seismic signal subsequent to the emission of the        seismic waves and to their propagation in the subsoil, the        gathered seismic signal comprising a reception sequence        corresponding respectively to each emission sequence; and    -   processing the seismic signal, the processing of the seismic        signal comprising, for each reception sequence corresponding to        an emission sequence, a summation of several terms which include        the seismic signal of said reception sequence and the seismic        signal of said reception sequence delayed by ΔT=2D·cos θ/V,        where V is the speed of propagation of the seismic waves in the        water and D is the depth associated with said emission sequence.

An emission sequence of the seismic waves can consist of a single firingperformed with the seismic source submerged at a depth d equal to thedepth D associated with this emission sequence (d=D).

Another possibility is that an emission sequence is composed of ksuccessive firings (k>1) positioned temporally with respect to oneanother according to the principle set forth above so as to reproducethe behavior of a virtual source of depth D. The k firings include inthis case a first firing performed with a seismic source submerged at adepth d₁ and k−1subsequent firings with seismic sources submerged atrespective depths d₂, . . . d_(k). The k firings of this emissionsequence are coordinated in such a way that, for each integer i lyingbetween 2 and k, the i^(th) firing of the emission sequence is performedwith a delay of (d_(i-1)+d_(i))·cos θ/V with respect to the (i−1)^(th)firing of the sequence. The depth D associated with such an emissionsequence is then the sum of the k depths d₁, d₂, . . . , d_(k).

In the particular case where the k coordinated firings are performed byone or more sources at one and the same depth d (d₁=d₂= . . . =d_(k)=d),we have D=k·d.

In an embodiment, seismic sources are submerged at n different depths,where n is an integer greater than 1, and several independent seismicwave emission sequences are successively produced with the aid of theseseismic sources and are associated with different depths D. It is inparticular possible to produce 2^(n)−1 independent seismic wave emissionsequences with the aid of the seismic sources submerged at n differentdepths, these 2^(n)−1 emission sequences including, for each integer ilying between 1 and n, C_(n) ^(i)=n!/[i!(n−i)!] independent emissionsequences each consisting of i coordinated firings from i sourcessituated at different depths.

In an embodiment, the summed terms for a reception sequence comprise theseismic signal of this reception sequence and k−1 copies of this sameseismic signal having delays respectively equal to i.ΔT for i=1, 2, . .. , k−1, where k is an integer greater than 1.

The ghost of the primary emission sequence is thus canceled, at thelevel of the processing of the signal, so as to simulate the behavior ofa source of depth k·D.

The summation can be further extended by contriving matters so that thesummed terms for a reception sequence comprise n times the seismicsignal of this reception sequence and, for each integer i lying between1 and n−1, n−i copies of this same seismic signal delayed by i.ΔT, wheren is an integer greater than 1. In the absence of noise and assuming thereflections to be perfect, the processing of the signal then makes itseem as if seismic waves had been emitted from n sources situated atdepths D, 2D, 3D, . . . , n.D, while in fact making do with a singlefiring, actual or virtual, at the depth D.

Noise may limit the performance of the above method. To remedy this,provision may be made for the emission of the seismic waves to compriseseveral (for example from 5 to 20) independent emission sequences eachassociated with a respective depth D₁, D₂, . . . D_(p), where p>1 is thenumber of sequences. In the particular case where D₁=D₂= . . . =D_(p)=D,the independent emission sequences are carried out at the same depth.The gathering of the seismic signal can then comprise the recording of preception sequences respectively subsequent to the p emission sequences,and the processing of the seismic signal can comprise the respectivesummation of said terms for each of the p reception sequences recordedand a combining of the p sums obtained.

An embodiment of the method for acquiring seismic data furthermorecomprises:

-   -   measuring the seismic waves emitted by a hydrophone submerged        under the seismic source or sources while being aligned along        the direction of angle θ;    -   applying to the seismic waves measured by the hydrophone a        summation processing identical to that applied to the seismic        signal;    -   verifying a convergence criterion on a signal resulting from the        processing applied to the seismic waves measured by the        hydrophone; and    -   stopping the emission sequences when the convergence criterion        is satisfied.

Another aspect of the present invention pertains to a device forprocessing a seismic signal gathered subsequent to the emission of theseismic waves with the aid of at least one submerged seismic source andto the propagation of the seismic waves in the subsoil, the seismicwaves having been emitted along a direction of emission forming an angleθ with the vertical and in the form of at least one emission sequenceassociated with a respective depth, the gathered seismic signalcomprising a reception sequence corresponding respectively to eachemission sequence. The device comprises a processor for summing severalterms which include the seismic signal of a reception sequencecorresponding to an emission sequence and the seismic signal of thissame reception sequence delayed by ΔT=2D·cos θ/V, where V is the speedof propagation of the seismic waves in the water and D is the depthassociated with said emission sequence.

Yet another aspect of the present invention pertains to a computerprogram for such a seismic signal processing device, the programcomprising instructions for, when it is executed on a processor of saiddevice, summing several terms which include the seismic signal of areception sequence corresponding to an emission sequence and the seismicsignal of this same reception sequence delayed by ΔT=2D·cos θ/V, where Vis the speed of propagation of the seismic waves in the water and D isthe depth associated with said emission sequence. There is furtherproposed a recording medium readable by computer, on which such acomputer program is recorded.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the present invention will becomeapparent in the description hereinafter of a nonlimiting exemplaryembodiment, with reference to the appended drawings in which:

FIGS. 1 and 2, previously commented on, are diagrams illustrating aphase of acquiring seismic data in a maritime environment;

FIGS. 3 and 4, previously commented on, are graphs showing spectragenerated along a direction θ with the aid of seismic sources submergedat various depths, with respective reflection coefficients of −1 and−0.7 at the surface;

FIG. 5 is a diagram analogous to that of FIG. 2, illustrating a phase ofacquiring seismic data in a maritime environment with two sources atdifferent depths;

FIG. 6 is a diagram analogous to that of FIG. 5, illustrating a phase ofacquiring seismic data in a maritime environment with three sources atdifferent depths;

FIG. 7 is a graph showing spectra generated along a direction θ with theaid of two seismic sources submerged at different depths;

FIG. 8 is a graph showing spectra generated along a direction θ with theaid of a submerged seismic source triggered in a repetitive manner;

FIG. 9 is a diagram illustrating a possible configuration for anacquisition of seismic data using the present invention;

FIG. 10 is a graph showing spectra generated with the aid of a seismicsource submerged at a depth of 5 m and triggered in a repetitive mannerin accordance with various embodiments of the invention, without takingaccount of noise;

FIG. 11 is a graph showing a magnified part of FIG. 10;

FIG. 12 is a graph showing spectra generated with the aid of a seismicsource submerged at a depth of 3 m and triggered in a repetitive mannerin accordance with various embodiments of the invention, without takingaccount of noise;

FIGS. 13A-C are graphs showing spectra analogous to those of FIG. 12,calculated while taking account of additive noise;

FIGS. 14A-C are graphs showing magnified parts of FIGS. 13A-C,respectively;

FIGS. 15A-C are graphs showing noisy spectra analogous to those of FIGS.13A-C, calculated while taking into account various realizations of theadditive noise; and

FIGS. 16A-C are graphs showing magnified parts of FIGS. 15A-C,respectively.

DETAILED DESCRIPTIO OF THE DRAWINGS

Returning to equation (1) hereinabove, it is seen that by repeating thefiring at the same source position with a delay ΔT, a seismic amplitudeS₂(t) given by:

S ₂(t)=S ₁(t)+S ₁(t−ΔT)=S(t)−S(t−2ΔT)  (2)

is generated along the direction θ.

From the point of view of the ghost phenomenon, the repetition of thefiring with the appropriate delay amounts to creating a virtual sourceof depth 2 d.

The process can be repeated to simulate a source of depth k.d byrepeating k−1 times the initial firing from the same depth with delaysi. AT for i ranging from 1 to k−1:

S _(k)(t)=S ₁(t)+Σ_(i=1) ^(k−1) S ₁(t−i·ΔT)=Σ_(i=0) ^(k−1) S₁(t−i·ΔT)=S(t)−S(t−k·ΔT)  (3)

Let us consider on the other hand two sources 10 ₁, 10 ₂ with the samecharacteristics, with respective depths d₁, d₂ and with the samehorizontal position (or situated in the same Fresnel zone). If these twosources 10 ₁, 10 ₂ are triggered with, for the second source 10 ₂, adelay (ΔT₁+ΔT₂)/2 =(d₁+d₂)·cos θ/V with respect to the first source 10₁, then the second source is synchronized with the arrival of the ghostof the first along the direction θ. In the above expression,ΔT_(i)=d_(i)·cos θ/V designates the delay of ghost associated with asource of depth d_(i). Along the direction θ, the primary emission ofthe second source 10 ₂ cancels the ghost of the first source 10 ₁.

This is what is illustrated in FIG. 5 which is similar to FIG. 2 withtwo sources 10 ₁, 10 ₂ whose ghosts are symbolized as 10′₁, 10′₂. Theprimary emission of the second source 10 ₂ erases the ghost of the first10 ₁, and it is as if the ghost were delayed by ΔT₁+ΔT₂, instead of ΔT₁if there had been the first source 10 ₁ only. The seismic amplitudeS₂(t) generated along the direction θ with the two sources 10 ₁, 10 ₂thus shifted in time is given by:

S ₂(t)=S(t)−S(t−(ΔT ₁ +ΔT ₂))=S(t)−S[t−2(d ₁ +d ₂)·cos θ/V]  (4)

It is noted that equation (2) is a particular case of equation (4) whend₁=d₂=d.

Equation (4) is generalizable to the case of k sources 10 ₁, 10 ₂, 10_(k) of respective depths d₁, d₂, . . . , d_(k), where k designates anarbitrary integer greater than 1. This is what is illustrated in FIG. 6in the particular case where k=3 and θ=0, with ghost sources 10′₁, 10′₂,. . . , 10′_(k) above the level M of the sea. If, for each integer isuch that 2≦i≦k, the i^(th) source 10 _(i) performs a firing at itsemission position of depth d_(i) with a delay(ΔT_(i-1)+ΔT_(i))/2=(d_(i-1)+d_(i))·cos θ/V with respect to the(i−1)^(th) firing performed by the (i−1)^(th) source 10 _(i-1) at theemission position of depth d_(i-1), the ghost is deferred in time so asto generate along the direction θ a seismic amplitude S_(k)(t) given by:

S _(k)(t)=S(t)−S[t−2(Σ_(i=1) ^(k)d_(i))·cos θ/V]=S(t)−S[t−Σ _(i=1) ^(k)ΔT _(i)]  (5)

With the k sources 10 ₁, 10 ₂, . . . . , 10 _(k) thus coordinated, it isas if a virtual source had been placed at the depth D=Σ_(i=1) ^(k)d_(i).

It may further be noted that equation (3) is a particular case ofequation (5) when d₁=d₂= . . . =d_(k)=d, and that equation (4) is aparticular case of equation (5) when k=2.

FIG. 7 shows spectra obtained by calculation in a similar manner tothose of FIG. 3 (θ=0). Curve 31 corresponds to the spectrum emitted froma source 10 ₂ submerged at the depth d₂=5 m, curve 32 to the spectrumemitted from a source 10 ₁ submerged at the depth d₁=10 m, and curve 33to the spectrum that would be emitted by a virtual source at a depthd₁+d₂=15 m. This spectrum 33 is that which is obtained with a firstfiring from the source 10 ₁ of depth d₁=10 m followed by a second firingdelayed by (ΔT₁+ΔT₂)/2=(d₁+d₂)/V from the source 10 ₂ of depth d₂=5 m.

If one is capable of aggregating a first firing at the depth d₁=10 mwith a second firing at the depth d₂=5 m, and then a third firing(virtual) at the depth d₁+d₂=15 m, one obtains a spectrum according tocurve 34 which is the sum of curves 31, 32 and 33. On this spectrum 34,it is observed that the content at the very low frequencies is greatlyimproved and that the notches are plotted. This spectrum is much betterthan the spectrum 35 obtained by summing in a conventional manner(without shift) signals emitted independently from the sources 10 ₁ and10 ₂ (curve 35 is the sum of curves 31 and 32).

It is noteworthy that the virtual firing at the depth d₁+d₂=15 m hasbeen carried out without having to physically implement a source at thisgreater depth.

FIG. 8 is a graph analogous to that of FIG. 7 in the case of two sourceswith the same depth, that is to say d₁=d₂=d. Curve 41 is the same ascurve 32 of FIG. 7, that is to say representing the spectrum emittedfrom the source 10 submerged at the depth d=10 m. Curve 42 representsthe spectrum obtained by firing twice from this source 10, or from twocollocalized sources. This is the same spectrum that would have beengenerated on the basis of a single virtual source of double depth. Curve43 represents the spectrum that would be generated by aggregating thetwo firings from the source 10 (spectrum 41) and the firing from thevirtual source (spectrum 42). Again, an appreciable improvement in thespectrum at the very low frequencies is observed by comparing with curve44 which is the sum of the two firings at 10 m.

By “collocalized sources” is meant here two sources having the sameemission position, or two slightly shifted positions at the same depthd, that is to say situated in the same Fresnel zone in relation to thefrequency of the emitted seismic waves. At the low frequencies, thisFresnel zone has typical dimensions of several tens of meters.

On the basis of a seismic emission sequence which has been generated bysuperimposing several firings shifted in time in the manner indicatedhereinabove, it is possible to gather a seismic signal at the level ofone or more receivers. Various positionings of the receiver arepossible.

An acquisition geometry to which the method according to the inventionis very suited is represented in FIG. 9. The receiver 30 is situated ina well 20 which has been drilled at the sea floor F, and the firings areexecuted from a source 10 placed substantially vertically in line withthe well, that is to say with θ≈0. The method is then used to recordvertical seismic profiles (VSP) which, after a post-acquisitionprocessing known per se, advise regarding the geological formationsencountered by the seismic waves in the subsoil along the well betweenthe sea floor F and the position of the receiver 30 and beyond.

Any type of underwater seismic source 10 can be employed, for examplecompressed-air gun, explosive, etc. The receiver 30 is for example ageophone secured against the wall of the well 20.

The method is also applicable at sea to techniques of surface seismicsurveying, sources, on streamers (receivers composed of hydrophones)hauled by a boat or on geophones placed on the sea floor F to recordseismic waves which have propagated and reflected on geological bedsunder the sea. The angle θ may then possibly deviate somewhat from thevalue θ=0.

With reference to FIGS. 5 to 8, embodiments of the invention have beenpresented in which multiple firings judiciously positioned over timemake it possible to reduce the incidence of the problem caused by theghost in underwater acquisition. In other embodiments describedhereinafter, the elimination of the ghost, or at least the reduction inits effects, results at least in part from the processing of the signalpicked up by the receiver.

Before undertaking the conventional post-acquisition processing of ameasured seismic signal sequence, a first step of the processing thenconsists in superimposing several terms comprising temporally shiftedversions of the seismic signal measured by the receiver in the course ofthe sequence received.

The emission sequence of seismic waves giving rise to the receptionsequence thus processed is associated with a depth denoted D. Thisemission sequence can consist:

-   -   of a single firing from a source 10 placed at a depth d, as in        the configuration of FIGS. 1 and 2. We then have D=d;    -   of a number k>1 of firings from k sources 10 ₁, 10 ₂, . . . , 10        _(k) placed at respective depths d₁, d₂, . . . , d_(k) and        triggered with mutual delays equal to (d_(i-1)+d_(i))·cos θ/V.        We then have D=Σ_(i=1) ^(k)d_(i) and, in the particular case        where d₁=d₂= . . . =d_(k)=d, D=k·d.

If R(t) denotes the seismic signal measured by the receiver an instant tin a given reception sequence, the summation of this signal R(t) with acopy R(t−ΔT) of this same signal delayed by ΔT=2D·cos θ/V gives rise toa processed signal R₂(t) with the expression:

R ₂(t)=R(t)+R(t−ΔT)=[S ₁(t)+S ₁(t−ΔT)]*r(t)=S ₂(t)*r(t)  (6)

where S₂(t) is given by equation (2) hereinabove, r(t) is the responseof the probed environment which depends on the reflections undergone bythe seismic waves between the geological beds, and * designates thelinear convolution operation. In expression (6), no account has beentaken of the noise which is added to the signal.

In expression (6), it is seen that the summation of the two temporallyshifted versions of the received signal amounts to making it seem as ifa firing had been performed at the depth D and then repeated with thedelay ΔT, that is to say as if the seismic emission originated from avirtual source of depth 2D, with a ghost deferred in time as explainedpreviously.

The summation (6) can be extended to an arbitrary number k (k>1) ofcopies of the received signal R(t) temporally shifted by multiples ofΔT, namely R(t), R(t−ΔT), R(t−2ΔT), . . . , R(t−(k−1)ΔT):

R _(k)(t)=Σ_(i=0) ^(k−1) R(t−i·ΔT)=[Σ_(i=0) ^(k−1) S ₁(t−i·ΔT)]*r(t)=S_(k)(t)*r(t)  (7)

In the signal R_(k)(t) thus processed, where S_(k)(t) is given byequation (3) hereinabove, the ghost is deferred to the time k·ΔT insteadof ΔT in the signal R(t) as received, and the spectral content at thelow frequencies is improved.

Consequently, the reception of just the signal R(t)=R₁(t) makes itpossible, in the reception processing, to regenerate signals R_(k)(t)for any integer k ranging from 2 to an arbitrarily chosen number n.

On the basis thereof, it is possible to undertake a new summation tomake it seem as if seismic waves had been emitted from n sources ofrespective depths D, 2D, . . . , n·D:

R′(t)=Σ_(k=1) ^(n) R _(k)(t)=[Σ_(k=1) ^(n) S_(k)(t)]*r(t)=S′(t)*r(t)  (8)

Expression (8) can also be written in such a way that the summed termscomprise n times the seismic signal R(t) and, for each integer i lyingbetween 1 and n−1, n−i times the seismic signal delayed by i·ΔT:

R′(t)=S′(t)*r(t)=Σ_(i=0) ^(n−1)(n−i)·R(t−i·ΔT)  (9)

Expression (8) or (9) can further be written:

R′(t)=[Σ_(k=1) ^(n) [S(t)−S(t−k·ΔT)]]*r(t) =n·S(t)*r(t)−[Σ_(k=1) ^(n)S(t−k·ΔT)]*r(t)  (10)

where it is seen that the amplitude of the reflectivity associated witheach ghost is n times less significant than that associated with theprimary emission. The number n of firings taken into consideration can apriori be chosen as large as is desired. The hardware constraint ofhaving to multiply the firings at closely spaced time intervals from thesame emission position is therefore circumvented.

FIG. 10 shows, with amplitudes in decibels:

-   -   the spectrum of the signal emitted on the basis of a single        Dirac pulse from a source 10 of depth d=5 m (curve 51);    -   the spectrum resulting from the summation (8) or (9) pertaining        to n=10 sources comprising the source 10 generating the Dirac        pulse at the depth D=d=5 m and n−1=9 virtual sources of        respective depths 2d, 3d, . . . , 10d (curve 52);    -   the spectrum resulting from the summation pertaining this time        to n=100 sources of respective depths d, 2d, . . . , 100d (curve        53).

FIG. 11 is a magnification of the part at the lowest frequencies of FIG.10. Added thereto is the spectrum 54 obtained on the basis of anon-repeated firing at the depth d=5 m, but with a reflectioncoefficient of −0.7 at the water-air interface (equivalent of curve 22of FIG. 4).

It is apparent that onward of about n=10 firings, the quality of thespectrum emitted becomes, at the low frequencies, as good as or betterthan by implementing a screen aimed at greatly decreasing the reflectioncoefficient. For n≈100, the spectrum is remarkably flat, with a residualripple B of less than 2.5 dB and a gain of greater than 12 dB for afrequency of 1 Hz with respect to the spectrum 54.

It therefore appears desirable, generally, to choose in expression (8)or (9) a number n greater than 10, and preferably greater than 50.

As mentioned previously, no account is taken of the additive noise inexpressions (6)-(10). FIGS. 12-16 make it possible to observe the impactof noise on the proposed scheme. These figures show, up to the firstnotch, spectra of real seismic emissions (when n=1) or virtual seismicemissions (when n>1), expressed in decibels for a source 10 of depth d=3m.

FIG. 12 is analogous to FIG. 10, except for the difference that thedepth d is smaller (3 m rather than 5 m). No noise has been taken intoaccount in the calculations. The spectrum 61 corresponds to a singlefiring without repetition. The spectrum 62 corresponds to a singlefiring with repetitions up to n=10. The spectrum 63 corresponds to asingle firing with repetitions up to n=100.

In FIG. 13A, the spectrum 71 corresponds to a single firing withoutrepetition but with random noise. In FIGS. 13B and 13C, the spectra 72and 73 correspond to a single firing with repetitions up to n=10 and upto n=100, respectively, and with noise with the same variance (25%).

The noise noticeably degrades the quality of the spectrum. Its influenceon the lowest frequencies is visible in FIGS. 14A-C. In FIG. 14A (norepetition), curves 61 and 71 correspond to those representedrespectively in FIGS. 12 and 13A at the low frequencies, that is to sayless than 10 Hz. In FIG. 14B (repetition with n=10), curves 62 and 72correspond to those represented respectively in FIGS. 12 and 13B at thelow frequencies. And in FIG. 14C (repetition with n=100), curves 63 and73 correspond to those represented respectively in FIGS. 12 and 13C atthe low frequencies.

It is seen that though the repetition process flattens the spectra andreduces the width of the notches, it does not have any appreciableeffect on the amplitude of the noise when a single firing is physicallycarried out.

It is however possible to reduce the impact of the noise by resorting toseveral emission sequences which follow one another in an independentmanner. Here p denotes the number of successive and independent emissionsequences (p>1), and D₁, D₂, . . . D_(p) denote the depths respectivelyassociated with these p emission sequences. To each emission sequenceproduced by the seismic source or sources 10 there corresponds arespective reception sequence at the level of the geophone 30.

For example, p independent successive firings can be undertaken from thesource 10 submerged at the depth d and for each firing (a firing formingin this case an emission sequence) the n repetitions can be generated atthe stage of the processing applied to the reception sequences.

The successive firings, temporally spaced so as not to interfere withone another, give rise to the recording of respective receptionsequences R(t) that are each summed by the process (8) or (9) describedhereinabove, given a certain number n. The sums thus obtained forvarious sequences are thereafter combined to exploit successiveobservations affected by independent noise. The combination can againconsist of a summation.

The compressed-air source can be reloaded between two successivefirings. The number p of these firings typically lies between 5 and 20.For example, it may be from 8 to 10. This number p remains moderate, andmakes it possible to carry out the series of measurements in a fairlybrief time, the availability of the well 20 being limited in practicebecause of the operational drilling or production constraints.

In FIG. 15A, the spectrum 81, similar to the spectrum 71 of FIG. 13A,corresponds to a single firing without repetition (n=1), with noise. InFIGS. 15B and 15C, the noisy spectra 82 and 83 correspond to a singlefiring with repetitions up to n=10 and up to n=100, respectively, eachrepetition being generated by calculation by adding a differentrealization of the noise of the same variance. Stated otherwise, asignal S_(k)(t) has firstly been calculated according to (5) on thebasis of a Dirac pulse S(t)=δ(t) for k=1, 2, . . . , n, and then a noisysignal S_(k)(t)+N_(k)(t) has been calculated by adding a noise termN_(k)(t) drawn randomly for each integer k. The summation then givesrise to a seismic signal S′(t):

S′(t)=Σ_(k=1) ^(n) [S _(k)(t)+N _(k)(t)]  (11)

which, after convolution with the response r(t) of the environment,provides a noisy version of equation (8) whose Fourier transform isshown in FIGS. 15A-C.

FIGS. 15A-C evince not only a flattening of the spectrum but also anappreciable increase in the signal-to-noise ratio when the number ofrepetitions increases when resorting to different realizations of thenoise. In practice, the different realizations of the noise are obtainedusing p>1 real firings.

This increase in the signal-to-noise ratio with the number ofrepetitions is still sharper at the low frequencies. This may be seen inFIGS. 16A-C. In FIG. 16A (no repetition), curves 61 and 81 correspond tothose represented respectively in FIGS. 12 and 15A at the lowfrequencies, less than 10 Hz. Of course, a signal-to-noise ratiocomparable to that of FIG. 14A is found in FIG. 16A. In FIG. 16B(repetition with n=10), curves 62 and 82 correspond to those representedrespectively in FIGS. 12 and 15B at the low frequencies. And in FIG. 16C(repetition with n=100), curves 63 and 83 correspond to thoserepresented respectively in FIGS. 12 and 15C at the low frequencies.

It is therefore beneficial to use multiple firings on site and not tomake do with a single firing that would be repeated when processing.

However, there is nothing to prevent the carrying out of a certainnumber of firings during acquisition, making it possible to obtain asufficient signal-to-noise ratio, and to continue the artificialrepetition of these p firings during processing.

Thus, in an embodiment of the method according to the invention wherethe number of firings p is chosen according to circumstances, theseismic waves emitted from the source 10 are furthermore measured by ahydrophone 40 submerged under the seismic source in a position alignedalong the direction of angle θ. FIG. 9 shows such a hydrophone 40 in aVSP acquisition configuration (θ=0).

During acquisition, the seismic waves measured by the hydrophone 40 forma control signal W(t) which receives the same summation processing asthat which will be applied to the seismic signal R(t) recorded by thegeophone 30, for example according to (8) or (9) with a fairly largenumber n (for example n=100). This processing applied to the controlsignal W(t) also comprises the combining on the p firings which havealready been performed. As the acquisition proceeds, it is then possibleto examine whether the control signal W(t) thus processed after pfirings does or does not satisfy a convergence criterion.

If the convergence criterion is satisfied, the (real) firings arestopped and the well can be released to continue drilling or forproduction. If it is not satisfied, acquisition continues with anadditional firing, and so on and so forth. The total number of firingscan be limited to a maximum value, for example to p 32 10 or to p=20.

Several convergence criteria are usable.

The convergence criterion may in particular pertain to the spectralshape of the signal resulting from the processing applied to the controlsignal W(t). Accordingly, the spectrum of the combined signal iscalculated by Fourier transform and the amplitude of its ripples ismeasured in a span of low frequencies (for example from 0.5 to 20 Hz).If these ripples remain less than a threshold of a few decibels, it isdecided that the convergence criterion is satisfied and the firings arestopped; otherwise they are continued.

The criterion can further pertain to the signal-to-noise ratio of thesignal resulting from the processing applied to the control signal W(t).This ratio is calculated and if it is shy of a threshold, for example afew decibels, it is decided that the convergence criterion is satisfiedand the firings are stopped; otherwise they are continued.

The convergence criterion used can also combine a criterion on thespectral shape and another on the signal-to-noise ratio.

This type of embodiment with a control hydrophone 40 makes it possibleto ensure sufficient quality of the seismic waves utilized whileavoiding shutting down the well 20 for too long.

Some at least of the p emission sequences used to reduce the impact ofnoise can comprise several coordinated firings so as to produce theemission which would result from a firing from a virtual source (seeequation (5)). For one or more integers q lying between 1 and p, a depthD_(q)=Σ_(i=1) ^(k)d_(i) is then associated with the qth sequence withsources triggered successively at depths d₁, d₂, . . . , d_(k) and withthe appropriate delays between these triggerings.

If n real sources are employed at depths d₁, d₂, . . . , d_(n) toacquire VSPs, one possibility is to use up to:

-   -   C_(n) ¹=n emission sequences consisting of the independent        single firings from these n sources at the depths d₁, d₂, . . .        , d_(n),    -   C_(n) ²=n(n−1)/2 independent emission sequences each consisting        of 2 coordinated firings from 2 of the n sources;    -   C_(n) ^(i)=n!/[i!(n−i)49 ] independent emission sequences each        consisting of i coordinated firings from i of the n sources        (1≦i≦n);    -   and C_(n) ^(n)=1 independent emission sequence consisting of n        coordinated firings from the n sources.

It is thus possible to generate up to N=Σ_(i=1) ^(n)C_(n) ^(i)2^(n)−1independent emission sequences from varied depths with the aid of nsources only. The independent realizations of noise by which theseemission sequences are affected make it possible to increase thesignal-to-noise ratio. Through the repetition process implemented duringthe processing of the signal, it is possible to flatten their spectrumbetween the notches. Finally, a judicious choice of the depths of the nsources makes it possible to obtain a diversity not only in the noiseincluded in the emission sequences, but also in the depths of the realand virtual firings, thereby again helping to obtain a flat spectrum andto better circumvent the notches.

A seismic signal processing device usable to implement one or the otherof the above embodiments of the method according to the inventioncomprises one or more processors configured to sum the terms arisingfrom the signal R(t) measured in one or more reception sequences by oneor more seismic receivers in the manner described hereinabove.

The processing can be implemented with the aid of one or more computers.Each computer can comprise a calculation unit of processor type, amemory for storing data, a permanent storage system such as one or morehard disks, communication ports for managing communications withexternal devices, in particular for the loading of the signals R(t)recorded by one or more geophones 30, and user interfaces such as forexample a screen, a keyboard, a mouse, etc.

Typically, the calculations and the steps of the hereinabove describedmethod are executed by the processor or processors using softwaremodules which can be stored, in the form of program instructions or codereadable by the computer and executable by the processor, on a recordingmedium readable by computer such as a read only memory (ROM), a randomaccess memory (RAM), CD-ROMs, magnetic tapes, diskettes and opticaldevices for storing data.

The embodiments described hereinabove are illustrations of the presentinvention. Diverse modifications may be made to them without departingfrom the scope of the invention which emerges from the appended claims.

1. A method for acquiring seismic data relating to a subsoil zonesituated under the sea, the method comprising: emitting seismic wavesalong a direction of emission forming an angle θ with the vertical, theemitted seismic waves comprising at least one emission sequencegenerated with the aid of at least one submerged seismic source eachemission sequence having an associated depth; gathering a seismic signalsubsequent to emission of the seismic waves and to propagation of theseismic waves in the subsoil, the gathered seismic signal comprising areception sequence corresponding respectively to each emission sequence;and processing the seismic signal, wherein processing the seismic,signal comprises, for each reception sequence corresponding to anemission sequence, summing a plurality of terms including the seismicsignal of said reception sequence and the seismic signal of saidreception sequence delayed by ΔT=2D·cos θ/V, where V is a speed ofpropagation of the seismic waves in water and D is the depth associatedwith said emission sequence.
 2. The method for acquiring seismic data asclaimed in claim 1, wherein the summed terms for a reception sequencecomprise the seismic signal of said reception sequence and k−1 copies ofthe seismic signal of said reception sequence having delays respectivelyequal, to i·ΔT for i=1, 2, . . . , k−1, where k is an integer greaterthan
 1. 3. The raethod for acquiring seismic data as claimed in claim 1,wherein the summed terms for a reception sequence comprise n times theseismic signal of said reception sequence and, for each integer i lyingbetween 1 and n−1, n−i times the seismic signal of said receptionsequence delayed by i·ΔT, where n is an integer greater than
 1. 4. Themethod for acquiring seismic data as claimed in claim 3, wherein thenumber n is greater than
 10. 5. The method for acquiring seismic data asclaimed in claim 1, wherein an emission sequence of the seismic wavescomprises a single firing performed with the seismic source suhmerged atthe depth D associated with said emission sequence.
 6. The method foracquiring seismic data as claimed in claim 1, wherein an emissionsequence of seismic waves comprises k successive firings, where k is anInteger greater than 1, including a first firing performed with aseismic source submerged at a depth d₁ and k−1 subsequent firings withseismic sources submerged at respective depths d₂, . . . , d_(k),wherein the k firings of said emission sequence are coordinated in sucha way thai, for each integer i lying between 2 and k, the i^(th) firingof the emission sequence is performed with a delay of (d_(i-1)+d₁)·cosθ/V with respect to the (i-l)lh firing of the emission sequence, andwherein the depth D associated with said emission sequence is the sum ofthe k depths d₁, d₂, . . . , d_(k).
 7. The method for acquiring seismicdata as claimed in claim 6, wherein the firing depths are identical insaid emission, sequence, i.e, d₁=d₂= . . . =d_(k)=D/k.
 8. The method foracquiring seismic data as claimed in claim 6, wherein seismic sourcesare submerged at n different depths, where n is an integer greater than1, wherein a plurality of independent seismic wave emission sequencesare successively produced with the aid of said seismic sources and areassociated with different depths D.
 9. The method for acquiring seismicdata as claimed in claim 8, wherein 2^(n)−1 independent seismic waveemission sequences are successively produced with the aid of the seismicsources submerged at n different depths, the 2^(n)−1 emission sequencesincluding, for each integer i lying between 1 and % n, C_(n) ^(i)=n!/[i!(n−i)!] independent emission sequences each consisting of icoordinated firings from i sources situated at different depths.
 10. Themethod for acquiring seismic data as claimed in claim 1, wherein theemission of the seismic waves comprises p independent emission sequenceseach associated with a respective depth D₁, D₂, . . . D_(p), p being aninteger greater than 1, wherein gathering the seismic signal comprisesrecording p reception sequences respectively subsequent to said pemission sequences, and wherein processing the seismic signal comprisesrespectively summing said terms for each of the p reception sequencesrecorded and comhining the p sums obtained.
 11. The method for acquiringseismic data as claimed in claim 10, wherein the number p of emissionsequences is between 5 and
 20. 12. The method for acquiring seismic dataas claimed in respectively summing claim 10, further comprising:measuring the seismic waves emitted by a hydrophone submerged under eachseismic source and aligned along the direction of angle θ; applying tothe seismic waves measured by the hydrophone a summation processingidentical to that applied to the seismic signal; verifying a convergencecriterion on a signal resulting from the processing applied to theseismic waves measured by the hydrophone; and stopping the emissionsequences when the convergence criterion is satisfied.
 13. The methodfor acquiring seismic data, as claimed in claim 12, wherein theconvergence criterion relates to a spectral shape ofa signal resultingfrom the processing applied to the seismic waves measured by thehydrophone.
 14. The method for acquiring seismic data as claimed inclaim 1, wherein the direction of emission is vertical, that is to sayθ=0.
 15. A device for processing a seismic signal gathered subsequent toemission of seismic waves with the aid of at least one submerged seismicsource and to propagation of the seismic waves in the subsoil, theseismic waves having. been emitted along a direction of emission formingan angle θ with the vertical and in the form of at least one emissionsequence associated with a respective depth, the gathered seismic signalcomprising a reception sequence corresponding respectively to eachemission sequence, the device comprising a processor for summing aplurality of terms including the seismic signal of a reception sequencecorresponding to an emission sequence and the seismic signal of saidreception sequence delayed by ΔT=2D·cos θ/V, where V is a speed ofpropagation of the seismic waves in water and D is the depth associatedwith said emission sequence.
 16. The seismic signal processing device asclaimed in claim 15, wherein the summed terms for a reception sequencecomprise n times the seismic signal of said reception sequence and, foreach integer i lying between 1 and n−1 n−i times the seismic signal ofsaid reception sequence delayed by i·ΔT, where n is an integer greaterthan
 1. 17. The seismic signal processing device as claimed in claim 15,configured to process p reception sequences respectively subsequent to pemission sequences each associated with a respective depth D₁, D₂, . . ., D_(p), p being an integer greater than 1, the processor beingconfigured to respectively sum said terms for each of the p receptionsequences recorded and to combine the p sums obtained.
 18. The devicefor acquiring seismic data as claimed in claim 17, wherein the number pof reception sequences is between 5 and
 20. 19. A computer-readablestorage medium having a program stored thereon for running in a devicefor processing a seismic signal gathered subsequent to the emission ofseismic waves with the aid of at least one submerged seismic source andto propagation of the seismic waves in the subsoil, the seismic waveshaving been emitted along a direction of emission forming an angle θwith the vertical and in the form of at least one emission sequenceassociated with a respective depth, the gathered seismic signalcomprising a reception sequence corresponding respectively to eachemission sequence, the program comprising instructions, for, when theprogram is run in a processor of said device, summing, a plurality ofterms including the seismic signal of a reception sequence correspondingto an emission sequence and the seismic signal of said receptionsequence delayed by ΔT=2D·cos θ/V, where V is a speed of propagation ofthe seismic waves in me-water and D is the depth associated with saidemission sequence.
 20. (canceled)
 21. A method for emitting seismicwaves in a maritime environment along a direction of emission forming anangle θ with the vertical, with the aid of at least one submergedseismic source the method comprising: performing a first firing from afirst emission position submerged at a depth d₁; and performing a secondfiring from a second emission position submerged at a depth d₂, with adelay equal to (d₁+d₂)·cos θ/V with respect to the first firing, where Vis a speed of propagation of the seismic waves in a water.
 22. Themethod for emitting seismic waves as claimed in claim 21, wherein thefirst and second emission positions are collocated, or situated at oneand the same depth d in one and the same Fresnel zone in relation to theemitted seismic waves, i.e. d₁=d₂=d.
 23. The method for emitting seismicwaves as claimed in claim 21, comprising, for each integer i lyingbetween 2 and k, where k is an integer greater than 2 : performing ani^(th) firing from an i^(th) emission position submerged at a depthd_(i), with a delay equal to (d_(i-1)+d₁)·cos θ/V with respect to the(i−1)^(th) firing.
 24. The method for emitting seismic waves as claimedin claim 23, wherein the emission positions are collocated, or situatedat one and the same depth d in one and the same Fresnel zone in relationto the emitted seismic waves.
 25. The method for emitting seismic wavesas claimed in claim 21, wherein the direction of emission is vertical,that is to say θ=0.
 26. A method for acquiring seismic data relating toa subsoil zone situated under the sea, the method comprising: emittingseismic waves along a direction of emission forming an angle θ with thevertical, with the aid of at least one submerged seismic source;gathering a seismic signal subsequent to emisslon of the seismic wavesand to propagation of the seismic waves in the subsoil; and processingthe seismic signal, wherein emitting the seismic waves comprises ksuccessive firings, where k is an integer greater than 1, including afirst firing performed at a depth d₁ and k−1. subsequent firings atrespective depths d₂, . . . , d_(k), and wherein, for each integer ibetween 2 and k, the i^(th) firing is performed with a delay of(d_(i-1)+d_(i))·cos θ/V with respect to the (i−1)^(th) firing, where Vis a speed of propagation of the seismic waves in water.
 27. The methodfor acquiring seismic data as claimed in claim 3, wherein the number nis greater than
 50. 28. The method for acquiring seismic data as claimedin claim 12, wherein the convergence criterion relates to asignal-to-noise ration of a signal resulting from the processing appliedto the seismic waves measured by the hydrophone.